The technology described herein relates generally to heat transfer in gas turbine engines and more particularly to apparatus for cooling structures in such engines.
A gas turbine engine includes a turbomachinery core having a high pressure compressor, combustor, and high pressure turbine (“HPT”) in serial flow relationship. The core is operable in a known manner to generate a primary gas flow. The high pressure turbine includes annular arrays (“rows”) of stationary vanes or nozzles that direct the gases exiting the combustor into rotating blades or buckets. Collectively one row of nozzles and one row of blades make up a “stage”. Typically two or more stages are used in serial flow relationship. The combustor and HPT components operate in an extremely high temperature environment, and must be cooled by air flow to ensure adequate service life.
Cooling air flow is typically provided by utilizing relatively lower-temperature “bleed” air extracted from an upstream part of the engine, for example the high pressure compressor, and then feeding that bleed air to high-temperature downstream components. The bleed air may be utilized in numerous ways, for example through internal convection cooling or through film cooling or both. Preexisting usage of bleed air and other cooling air flows the air over rib rougheners, trip strips, and pin fins. When used for convection cooling, the bleed air is often routed through serpentine passages or other structures according to an overall source-to-sink pressure difference, which generates fluid velocity distributions and associated heat transfer coefficient distributions as the cooling air passes through them. Because bleed air represents a loss to the engine cycle and reduces efficiency, it is desired to maximize heat transfer rates and thereby use the minimum amount of cooling flow possible. For this reason heat transfer improvement structures, such as pin fins or turbulators may be employed as integral portions of the cooled interior component surfaces.
Conventional turbulators are elongated strips or ribs having a generally square, rectangular, or other symmetric cross-section, and are generally aligned transverse to the average bulk direction of flow in a channel or near the surface. The turbulators serve to periodically “trip” the boundary layer across the entire width of a flow passage at the component interior surface and thereby enhance mixing of the near wall and bulk flows, promote flow turbulence, and increase surface heat transfer coefficients. Cooling effectiveness may thereby be increased. One problem with the use of conventional turbulators is that a flow recirculation zone is present downstream of each turbulator. This zone causes particulates entrained in the cooling air to be circulated, further interact with surfaces or deposit, and build up behind the turbulator. This build-up results in an insulating layer which reduces heat transfer rates to the cooling flow by increasing thermal resistance.
In lieu of turbulators or in addition thereto, a conventional pin fin has a generally symmetric shape of constant cross section with height, such as round, elliptic, or square, and results in a stagnation region at its leading face and a flow separation and recirculation region, or wake, aft of the feature. The wake region in particular can be relatively large, serving to churn the flow, but also to collect particulates within the recirculation zones. Consequently, there is a need for a cooling promoting device that does not necessarily span an entire widthwise dimension of a flow passage, but at the same time promotes turbulent flow without the adverse effects of wakes caused by conventional pin fins.